This invention relates to the measurement of bulk density, and more particularly to an apparatus and method for measuring the bulk density of a sample using gamma radiation. The invention is especially suited for measuring the bulk density of relatively small xe2x80x9cfinite volumexe2x80x9d samples, and in particular, cylindrically shaped samples such as core samples or gyratory compacted specimens of asphalt paving material.
In the asphalt pavement construction industry, the cylinder is a common sample geometry. In laboratories, cylindrical samples are prepared, typically with a gyratory compactor, and various material properties are studied to determine the best mix design for a pavement. In the field, cylindrical samples are cored from test strips, newly constructed roads, or existing roads. The material properties of these samples are then used to evaluate whether the test strip or the new pavement meets the design criteria and whether the existing road is in good operating condition or in need of repairs. Among the material properties studied in the cylindrical asphalt samples, material bulk density or bulk specific gravity is an important property.
Currently, several methods are used for measuring the density of cylindrical samples: dimensional analysis, the water displacement method, the paraffin coated method, and the para-film-covered method. In each case, the bulk density of a sample is derived by, as in the definition, dividing the dry sample mass by the estimated sample volume. All methods require a balance with a sensitivity of 0.1 g. to measure the mass of the sample. In the dimensional analysis method, sample volume is determined from the radius and thickness (height) measurements. Here, many readings of the radius and thickness of the sample are made using a vernier caliper. The average values of the radius and the thickness are then used to calculate the sample volume. The other methods use the Archimedes Principle for determining the sample volume. These methods require a large container filled with clean water. The water,temperature should be monitored and controlled at a specific temperature, e.g. at 25xc2x0 C. At one stage of the test, the sample is kept immersed in water for approximately 4 minutes and the weight of the sample, while suspended in water, is recorded. In the xe2x80x9cparaffin-coatedxe2x80x9d method, after determining the dry weight of the sample, a thin coating of paraffin is applied to cover the entire surface area of the sample. Then, the sample is weighed again in air. Finally, the sample is weighed while immersed in water. The xe2x80x9cparafilm-coveredxe2x80x9d method is similar to the xe2x80x9cparaffin-coatedxe2x80x9d method, except that a film is used to wrap the sample. The water displacement test, which is the fastest of all three Archimedes methods, takes about 6 minutes. The single operator precision of the water displacement method is 0.0124 g/cm3 and the paraffin-coated method is less than 0.02 g/cm3. More details can be found in standards ASTM D 2726 for the water displacement method and ASTM D 1188 for the paraffin-coated method.
Because an asphalt mix is heterogeneous and granular in nature, there is no single density determination method to cover all mix designs. The decision on which density determination method to use depends on the aggregate size and whether it is an open or closed mix design. For example, for mixes using aggregate with a nominal aggregate size of 9.5 mm and a void content less than about 6%, the water displacement method provides reasonably accurate densities. For open-graded mixes with larger aggregates and coarser gradations, the water displacement method provides densities higher than the xe2x80x9ctruexe2x80x9d densities. For such samples, the industry recommends using paraffin-coated or para-film covered methods for density determination. However, paraffin-coated and para-film methods are time consuming and their accuracy of density determination have been found to be highly user dependent. Furthermore, for many mix designs, the dimensional analysis method overestimates the volume and yields lower density values than the other methods. However, literature has shown that dimensional analysis provides better estimates of the densities of highly permeable specimens.
With more and more roads being constructed using mixes of large aggregate sizes and/or open graded mix designs, there is a need for developing techniques for determining the density of cylindrical asphalt samples.
For many decades, gamma-ray based nuclear gauges have been successfully used to determine densities of asphalt pavements. This type of gauge provides a non-destructive test method and provides density measurements rapidly (e.g. within 1 to 4 minutes).
Various designs for gamma-ray based surface gauges, depth probes, and the like have been reported in literature and are available commercially. Single system surface density gauges, such as the Model 3400 series Surface Moisture-Density gauges available from applicant""s assignee, Troxler Electronic Laboratories, Inc., are designed to be placed on a surface, such as an asphalt pavement surface, and the density determination assumes a sample of relative large or xe2x80x9cinfinite volumexe2x80x9d in relation to the field of view of the gauge. Such gauges are not designed to reliably measure the bulk density of relatively small, xe2x80x9cfinite volumexe2x80x9d samples, such as cylindrically shaped samples of asphalt paving mix.
In commonly owned U.S. Pat. No. 5,151,601, a system is described by which a nuclear asphalt content gauge, such as the Troxler 3200 series asphalt content gauge, can be used to determine the asphalt content of cylindrical samples of asphalt paving mix. However, currently no commercial nuclear gauge is available in the market for determining the density of cylindrical specimens.
The present invention provides a nuclear density test instrument and method which is suited for measuring the bulk density of relatively small, xe2x80x9cfinite volumexe2x80x9d samples, and is particularly suited for measuring the bulk density of cylindrical specimens, such as cylindrical core samples or gyratory compacted specimens of asphalt paving mix.
In one embodiment, the density gauge of the present invention comprises a plurality of sources of gamma radiation positioned in spaced-apart relation from one another for emitting gamma radiation from a plurality of spaced-apart locations into a sample placed nearby and a detector mounted for receiving gamma radiation which has penetrated the sample. Means is provided cooperating with the detector for calculating the bulk density of the sample based upon the gamma radiation counts by the detector. In one embodiment, each of the sources of gamma radiation is preferably a point source, and the sources are preferably mounted to a source plate. The sources are preferably at least three in number and are arranged in a common plane. In a further more specific aspect, the gauge includes a sample holder configured to hold a cylindrically shaped sample, the sample holder being mounted adjacent to said source plate to orient the sources at spaced locations opposite a first end of the cylindrically shaped sample.
Preferably, the detector is an energy selective detector configured to detect gamma radiation in a predetermined energy spectrum, which desirably is within the range of from 0.1 MeV to 2 MeV. The detector may comprise a scintillation detector, and the system may include an analyzer connected to the scintillation detector for detecting gamma radiation in the desired predetermined energy spectrum. In one specific embodiment, each of the sources is a 137Cs gamma-ray source with a 0.662 MeV primary energy, and the predetermined energy spectrum measured by the gauge falls within the range of 0.25 to 0.73 MeV.